MODES OF PROGRESSION USED BY MAN. 129 



body, but impel it with a certain speed both upwards and 

 forwards. It is known that to give a mass a rising motion, 

 a greater effort must be exerted than would be sufficient 

 simply to sustain it. 



The duration of the pressures on the ground is less than in 

 walking; this brevity is proportional to the energy with 

 which the feet tread on the ground. These two elements, 

 force and brevity of pressure, increase generally with the 

 speed at which a person runs. The frequency of the foot- 

 falls increases also with the speed of the runner ; but among 

 the various kinds of running, there are some in which the 

 extent of space passed over in a given time depends rather 

 on the extent of each pace than on their number. 



The essential character of running is, as we have said, the 

 time of suspension during which the body remains in the air 

 between two foot-falls. Fig. 3 0 clearly shows the suspension, by 

 the interval which separates the descent of the curves of the 

 right foot from the ascent of the curves of the left foot, and 

 vice versa. The duration of this time of suspension seems to 

 vary but little in an absolute manner ; but if we compare it 

 with the speed of a runner, we see that the relative time 

 occupied by this suspension increases with the speed of the 

 course, for the duration of each tread diminishes in proportion 

 to this speed. 



How is this suspension of the body at each impulse of the 

 feet produced ? We might think, on first consideration, that 

 it is the effect of a kind of leap, in which the body is pro- 

 jected upwards in so violent a manner by the impulse of the 

 feet, that it would describe in the air a curve, in the midst of 

 which it would attain its maximum elevation from the ground. 

 In order to convince ourselves that such is not the case, let us 

 make use of the apparatus which registers the re-actions or 

 vertical oscillations of the body. 



In fig. 30 is seen (upper line O) the tracing of oscillations 

 in running. This trace shows us that the body executes each 

 of its vertical elevations during the downward pressure of the 

 foot, so that it begins to rise as soon as the foot touches the 

 ground ; it attains its maximum elevation at the middle of the 

 pressure of this foot, and begins to descend again, in order to 

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